Analog-to-digital (A/D) conversion circuits which utilize delta-sigma converters have several distinct advantages over other types of A/D converter circuits and are used in such applications as digital recordings of audio signals which require A/D conversion circuits which have excellent signal-to-noise (S/N) characteristics.
A type of noise inherent in any A/D conversion system is the noise which results from the quantization of the analog signal. While for many applications the quantization noise can be assumed to have a white spectral content, i.e., uncorrelated noise spectrum, in actuality the quantization noise signals can be correlated. This correlation can occur when a delta-sigma type of A/D converter enters a limit cycle, and during these times the output of the A/D converter will exhibit tones. These tones are very undesirable if they fall in the frequency band of interest.
While limit cycles occur most prominently in first order delta-sigma converters, higher order converters are not entirely free of the limit cycles. In the past dither signals have been added at various points in the delta-sigma converter loop in an attempt to break up the limit cycles. However, since the dither signal itself is deterministic, it cannot entirely eliminate the possibility of limit cycles.
Therefore it can be appreciated that an apparatus and method for attenuating or breaking up undesirable tones in the frequency band of interest resulting from the correlation of the quantization noise in a delta-sigma converter is highly desirable.